In a motor vehicle alternator, an on-board network voltage is regulated by controlling an intensity of the magnetic flux, obtained from an excitation winding of a rotor, and passing through the windings of a stator. This function is ensured by an excitation circuit which adjusts a value of an excitation current of the rotor, according to the variations of loads and of the speed of rotation of the alternator.
An example of this type of excitation circuit is provided in the French patent application FR2733097 by the company VALEO EQUIPEMENTS ELECTRIQUES MOTEUR.
In order to limit losses by dissipation efficiently, the switching of an electronic power stage of this circuit is controlled at a fixed or variable frequency. A voltage at the terminals of the excitation winding is taken to the potential of the battery for a given time, then released until the end of the excitation period. In an established arrangement, a mean value of an excitation current is proportional to the ratio of a time in the high state to a period of an excitation signal. This duty cycle is determined by a control loop of a regulator.
When the excitation is released, the current in the rotor winding cannot be interrupted instantaneously.
The electronic power stage must provide a conduction path, in order to allow the rotor current to circulate freely. This function is conventionally carried out by a so-called “free wheel” diode which is placed in parallel on the excitation winding, and is activated naturally when the voltage at the terminals of the rotor becomes negative.
In order to prevent effects caused by the environment, such as corrosion or the appearance of saline bridges, the rotor winding is generally connected to the neutral potential of the alternator. The excitation stage of a regulator is thus constituted by:                a power transistor which is connected between a positive terminal of the alternator, and a positive terminal of the excitation winding;        a diode which is connected to the terminals of the excitation winding;        an excitation control circuit.        
In the case of integration in an integrated circuit, which at present mostly uses BCD technology, a first, high side switching element is a first power transmitter generally of the NMOS type. The free wheel diode is produced by using the base-emitter diode of a bipolar transistor. The disadvantage of the diode is to generate Vf*If*Do losses.
Certain technologies use the intrinsic diode of an NMOS, with the transistor being maintained in the blocked state. Intrinsic diodes generally have less good performance than bipolar diodes.
A first control circuit of a first gate voltage of the first, high side transistor forms the interface with the logic of the control loop. This first control circuit must generate switching gradients so as to:
control the form of the current switching in order to limit the EMC emissions on the on-board network;
optimise the switching time in order to limit the switching losses.
Examples of gate control strategies which carry out this control are known in the prior art.
Since a second, low side switching element is available in BCDMOS technology, it is also known for persons skilled in the art to control it in opposition to the first, high side excitation transistor, such as to switch it on during the free wheel phases. The advantage of this solution is to have low conduction losses, associated with the decrease in the equivalent resistance (RDSon) between a source and a drain of the second, low side transistor.
However, in order to avoid simultaneous conduction of the two transistors, a control circuit must transmit control signals of the first gate voltage, and of a second gate voltage of the second transistor, which do not overlap. The control circuit must therefore generate dead times between the activations of the transistors.
A control circuit of this type also has the disadvantage of having a common mode between the commands of the high side and low side power transmitters, in particular during EMC tests, which can make it difficult to use in the motor vehicle industry.